Highly corrosive environments require the use of materials which are able to withstand corrosive attack from the particular environment for extended periods of time. For example, blades and other components in turbines used to generate electrical power from steam recovered from geothermal sources must be able to function in an environment containing high concentrations of sulfur dioxide, chloride ions and other highly corrosive materials.
Further, chemical reaction vessels, pipes leading to them, and similar apparatus are sometimes exposed to highly corrosive acid solutions, such as concentrated nitric acid. Stainless steels are commonly used for the construction of such equipment, but even they do not have sufficient corrosion resistance under certain circumstances.
Corrosion-resistant coatings of amorphous alloys of stainless steel are presently available for the protection of substrates which are subject to corrosive attack by their environment. Most of these alloys are stabilized in the amorphous state by one or more of the metalloid elements such as B, C, Si and P. Our patent applications Ser. Nos. 360,117 and 060,759, now U.S. Pat. Nos. 4,496,635 and 4,786,468, respectively, describe enhanced amorphous coatings for rendering a substrate highly corrosion resistant. These two patents are hereby incorporated by reference.
The coating described in the U.S. Pat. No. 4,496,635 is capable of remaining amorphous at temperatures up to 400.degree. C. It consists essentially of the formula M.sub.a Cr.sub.b T.sub.c, where "M" is at least one element selected from the group consisting of iron and nickel, "T" is at least one element selected from the group consisting of tantalum, titanium, zirconium, hafnium, niobium, molybdenum, and tungsten. Quantity "a" is 35-75 mole percent, "b" is 5-20 mole percent, "c" is 5-55 mole percent, and "b" plus "c" is equal to at least 25 mole percent.
U.S. Pat. No. 4,786,468 describes a coating consisting essentially of an alloy of stainless steel and at least one of tantalum or tungsten present in a range of from 60-90 mole %.
Examples in these patents describe depositing of such glassy stainless steel coatings by sputter deposition in small scale experiments (less than or equal to 0.1 m.sup.2 area substrates). Sputter deposition requires a high vacuum environment and typically achieves a low deposition rate. It may be prohibitively expensive to sputter deposit onto large surfaces or to a large number of parts where coating thicknesses need to be between 25-250 microns.
Plasma spraying of alloy coatings is also recognized as an application method in the prior art. Such processes when applied to materials that readily oxidize such as refractory metal alloys, generally require plasma spraying in a low pressure atmosphere (vacuum) or in the presence of an inert gas. For example, studies of plasma sprayed Ta, Nb, Ti, and WC stress the need for an inert gas atmosphere or vacuum to obtain dense, high purity coatings. See for example, E. Lugscheider et al., "Vacuum Plasma Spraying of Tantalum and Niobium", J. Vac. Sci. Tech. A3 (1985) 2469-2473; H. D. Steffens et al.; "A Comparison of Low Pressure Arc and Low Pressure Plasma Sprayed Titanium Coatings", J. Vac. Sci. Tech. A3 (1985) 2459-2463; and M. E. Vinayo et al., "Plasma Sprayed Sc-Co Coatings: Influence of Spray Conditions (Atmospheric and Low Pressure Plasma Spraying) on the Crystal Structure, Porosity, and Hardness", J. Vac. Sci. Tech. A3 (1985) 2483-2489. Apparently good WSi.sub.2 coatings have been produced in an open oxygen containing atmosphere, but the coatings were not significantly amorphous. See for example, O. Knotek et al., "On Plasma Sprayed WSi.sub.2 and Cr.sub.3 C.sub.2 -Ni Coatings", J. Vac. Sci. Tech. A3 (1985) 2490-2493.
Using an inert gas or a vacuum atmosphere for plasma spraying adds to inconvenience and cost for refractory metal alloy coating process. This invention overcomes these and other problems associated with plasma spraying of coatings onto substrates.